Atomic layer processes (ALP) are thin film fabrication techniques where the physical and/or chemical interactions occur at an atomic scale in discrete steps. Atomic layer deposition and atomic layer etching are examples of atomic layer processes that are used for controlled material growth and controlled etching of material, respectively.
The atomic layer deposition (ALD) technique, formerly known as atomic layer epitaxy, is widely used in semiconductor fabrication and other related industries for deposition of oxides (TiO2, HfO2, ZrO2, SiO2, etc.), nitrides (TiN, HfN, ZrN, SiN, etc.), and other compound thin films. The ALD technique has also been used for deposition of organic materials or organic-inorganic hybrid materials, where ALD is referred to as molecular layer deposition, or molecular layer epitaxy.
The atomic layer etching (ALE) technique is relatively recent as compared to ALD, but is considered to be a most promising etching process in fabrication of sub-10 nm semiconductor devices.
Typically, an ALP process may involve one or more precursors, where each precursor is sequentially pulsed into a process reactor, and thereby onto a substrate, whereon the precursor species undergoes a respective surface reaction. In order to ensure a precursor reaction proceeds until saturation, an exceedingly large number of precursor molecules are introduced in each pulse.
FIG. 1 illustrates a typical ALP process using two precursors, precursor-A 100 and precursor-B 110, where the precursors (100, 110) are introduced into a process reactor as discrete pulses. As shown, during the process, the precursor-A 100 is released into the process reactor as pulse-A 120 of duration 130 followed by an inert gas purge or reactor evacuation for duration 140. Similarly, the second precursor-B 110 is released into the reactor as pulse-B 150 of duration 160 also followed by an inert purge or reactor evacuation for duration 170. The precursor pulses (120, 150) and the intermittent inert gas purges or reactor evacuation steps (140, 170) constitute a “process cycle” with a total cycle-duration of 180. The process cycle is referred to as a “deposition cycle” in ALD, and as an “etching cycle” in the case of ALE. The pulse durations (130, 160) are kept excessively long in order that the reaction of precursors (100, 110) at the substrate attains a self-limiting surface saturation condition. In addition, the purge or reactor evacuation durations (140, 170) are required to be sufficiently long enough to ensure excess un-reacted precursor molecules from pulses (120, 150) along with the by-products of their respective surface reactions are totally pumped out of the process reactor.
The fundamental self-limiting characteristics of ALP processes ensure uniform precursor reactions over large area substrates. Furthermore, as these surface reactions occur at the atomic scale, ALP processes may be precisely controlled with the number of “process cycles”. These exceptional features make ALP processes an indispensable technique for fabrication of critical features in semiconductor devices and related applications. For example, ALD films are highly uniform in thickness, exhibit excellent step-coverage within high-aspect ratio non-planar features, and the “growth per deposition cycle” in range of 0.01-0.10 nanometers/cycle enables for a thickness control within ±0.10 nanometers.
Although ALP processes exhibit several advantages over other thin film processing techniques, the efficiency (η) of precursor utilization, defined as the ratio of the number of precursor molecules involved in ALP surface reactions to the total number of precursor molecules pulsed into the reactor, remains a major concern. Since the ALP surface reactions are self-limiting or self-terminating, excess precursor molecules are pumped out of the reactor as waste, which results in poor precursor utilization efficiency. The poor precursor utilization efficiency η of the ALP process directly translates into high operation-costs, since the cost of electronic grade high-purity precursors is expensive and is a major component of the process.
FIGS. 2a and 2b illustrate examples of an effusion-mode and a displacement-mode of precursor delivery systems, respectively, as used in prior art ALP processes.
In the effusion-mode precursor delivery system as shown in FIG. 2a, a precursor ampoule 200 is installed onto the precursor delivery line 210 through a three-port valve 220. In the OFF state, the valve 220 restricts the precursor flow 230 into the carrier gas stream 212 flowing into the ALP process reactor. Over a precursor pulse duration, when the valve 230 is switched to the ON state (as shown), the effusion of precursor molecules from ampoule 200, results in precursor flow 230 into the carrier gas stream 212, and the released precursor molecules are delivered to the ALP reactor.
In the displacement-mode precursor delivery system as shown in FIG. 2b, the precursor ampoule 240 is installed onto the precursor delivery line 250 by two three-port valves (260, 264), and the delivery line 250 is further equipped with a bypass valve 262. If valves 260 and 264 are turned OFF and valve 262 is turned ON, carrier gas flow 252, is constrained to the flow-path 254 thereby restricting flow of precursor molecules into the reactor. However, when the valves 260, and 264 are turned ON and valve 262 is turned OFF, the carrier gas flow 252 follows the flow-path 258 where the carrier gas displaces the precursor vapor from ampoule 240 into the delivery line 250, and these displaced precursor molecules are then delivered into the ALP process reactor.
The rate at which precursor molecules are delivered into the reactor during a precursor pulse, depends upon the precursor vapor pressure within the ampoule, which further depends upon the ampoule temperature. In prior art ALP processes, in order to attain the characteristic self-limiting surface saturation condition within practically short times, the precursor ampoule(s) are typically maintained at ambient (or room) or elevated temperatures.
Furthermore, in prior art ALP processes, in order to improve precursor utilization efficiency, the precursor pulses shown as 120 and 150 in FIG. 1, are made short by reducing their respective pulse duration 130 and 160. In the prior art ALP reactors, the precursor molecules could be delivered as a pulse of ≤10 milliseconds using fast-switching valves shown as 220 in FIGS. 2a and 260, 262, 264 in FIG. 2b. However, shortening precursor pulses adversely affects the residence time of the pulsed precursor molecules within the ALP reactor. For example, in the prior art self-limiting ALD process reactor, precursor utilization efficiency (η) is typically approximately 0.01 (or 1%).
The increasing application of ALP processes in the semiconductor industry has increased the need for methods to improve utilization efficiency of high-purity precursors. Thus, there exists a need for alternative strategies for significantly improving precursor utilization efficiency in ALP processes, to make ALP techniques more acceptable in large volume fabrication.